Generation of high-power spatially-restructurable spectrally-tunable beams in a multi-arm-cavity vecsel-based laser system

ABSTRACT

A collinear T-cavity VECSEL system generating intracavity Hermite-Gaussian modes at multiple wavelengths, configured to vary each of these wavelengths individually and independently. A mode converter element and/or an astigmatic mode converter is/are aligned intracavity to reversibly convert the Gaussian modes to HG modes to Laguerre-Gaussian modes, the latter forming the system output having any of the wavelengths provided by the spectrum resulting from nonlinear frequency-mixing intracavity (including generation of UV, visible, mid-IR light). The laser system delivers Watt-level output power in tunable high-order transverse mode distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation from U.S. patent applicationSer. No. 16/682,284 filed on Nov. 13, 2019 and now published as US2020/0083666, which is a continuation from U.S. patent application Ser.No. 16/092,379, filed Oct. 9, 2018 and now granted as U.S. Pat. No.10,511,145. The U.S. patent application Ser. No. 16/092,379 is anational phase of the International Patent ApplicationPCT/US2017/067359, filed Dec. 19, 2017, which in turn claims priorityfrom U.S. Provisional Patent Applications No. 62/437,452 (filed on Dec.21, 2016) and No. 62/569,891 (filed on Oct. 09, 2017). The disclosure ofeach of the above-identified patent documents is incorporated byreference herein.

RELATED ART

In various contexts, optics as technology enabling the development ofmany industrial fields can be greatly enriched with the ability toproduce a variety of novel laser beams that include Hermite-Gaussian(HG), Laguerre-Gaussian (LG), Bessel, Airy, and Helmholtz beams, to namejust a few.

The Laguerre-Gaussian beams—that is, laser beams having a transversedistribution of optical power (interchangeably referred to herein astransverse modes) described by LG functions—are probably the mostprolific realizations in practice. The beams with LG transverse modeshave been used to enhance the information capacity of both classical andquantum communication channels by employing many orthogonal opticalangular momentum (OAM) modes as opposed to two polarization modes. Otherkey applications of the LG modes derive from the ability of the lightOAM to be transferred to and rotate particles such as atoms andnanoparticles, driving optical micro-machines, and excitation ofmatter-wave vortices in cold atomic gases. Still other applicationsarise in quantum entanglement of twisted photon states and quantuminformation processing, spiral phase contrast microscopy, andastrophysics and space physics studies. Overall, it is rather undisputedthat the ability to produce beams having diverse LG modes will continueto have significant impact in the fields of quantum communication, whereLG beams are used to increase the information capacity, laser trappingwhere the laser can provide novel trapping potentials, and laser writingof optical structures such as extended waveguides using Bessel beams. Itis of significant interest to have a source of novel laser beams withhigh power and over a large range of wavelengths for the variety ofapplications.

Despite familiarity with LG modes in applied and basic science, nounified optical source configured to generate laser beam(s) in suchmodes at high-power levels while permitting for spectral tunability ofthe laser beam(s) exists up to-date: a few known attempts to generate LGlaser beams produced results limited in power and/or operatingwavelength range. One means of generating LG beams related art refers tomakes use of a spatial light modulator (SLM) device (such as one builtaround a MEMS structure), which is known to operate at wavelength(s)falling within the range of about 400 -1064 nm and limited to being ableto handle a few Watts of power before damage of the individualreflectors of the SLM device sets in. While higher power LG beams couldarguably be realized using fused silica phase-masks, a different phasemask is necessary for each realization of the OAM. Likewise, althoughspecific lasers can be coaxed to operate on LG modes, such a solutionimposes a limit on the spectral tenability of the produced LG-laserbeams and requires a multiplicity of the laser sources that inevitablydrives up the costs of the overall system.

Despite significant interest in generation of laser light in LG modes,the generation of high power, high efficiency beams remains a majorchallenge. Implementations of the present invention address anunaddressed need in developing a unified source or generator of lightcharacterized by LG modes, and are expected to have a transformativeimpact on the multiple applications described above, and lead tohitherto unforeseen new uses as new wavelengths and powers are openedup.

SUMMARY

Embodiments of the invention provide a laser source that includes (i) alaser cavity network, including first and second spatially-distinctcavity arms and a collinear portion, (ii) a first optical system,disposed across an axis of the at least one of the first and secondcavity arms, to either refract or reflect light incident on the firstoptical system while, at the same time transforming a transversedistribution of such light, and (iii) a second optical system disposedacross this axis and characterized by optical losses, at thecorresponding wavelength, that are non-uniformly distributed across thesecond optical system, the second optical system disposed between thecorresponding gain medium and the first optical system.

Here, the first and second spatially-distinct cavity arms share thecollinear portion, and at least one of the first and second cavity armscontains (intracavity) a corresponding gain medium including one of (i)a VECSEL-based laser gain medium, (ii) a solid-state gain medium, and(iii) a fiber amplifier and configured to provide amplification of lightat a corresponding wavelength. (The discussion below refers only to theVECSEL-based gain medium, for simplicity of disclosure). The lasersource if configured to maintain, in operation, generation of lighthaving, intracavity, (a) a first transverse mode distribution in a firstportion of the laser cavity network between a gain medium and the secondoptical system; (b) a second transverse mode distribution in a secondportion of the laser cavity network between the second and first opticalsystems, and (c) a third transverse mode distribution in a third portionof the laser cavity network, the third portion being a remaining portionof the laser cavity network. In a specific case, such first, second, andthird transverse mode distributions are different from one another.

Embodiments of the invention additionally provide a laser sourceincluding a laser cavity network that contains i) at least one outputcoupler (configured to transmit at least a portion of light generatedintracavity by the laser source to form a light output; ii) multiplespatially-distinct cavity arms; iii) a collinear cavity portion; and(iv) an astigmatic mode converter (AMC). The AMC may be disposedintracavity or outside of the cavity network. Here: each of a first armand a second arm is configured to support intracavity circulation oflaser light at at least one wavelength; the first and second arms sharethe collinear cavity portion and overlap with one another along thecollinear cavity portion; and at least one of said first and second armscontains an intracavity mode converter element (MCE). The light outputfrom the laser source has a transverse mode distribution that is tunablein response to a change in spatial cooperation between a) the at leastone of said first and second arms and b) at least one of the MCE andAMC. The laser source may optionally include at least one wavelengthtuning mechanism disposed within the cavity network and configured totune a wavelength of light in at least one of the first and secondcavity arms. Optionally, at least one of the MCE and AMC is disposed ina collinear portion of the laser cavity network (i) to cause a formationof light having a Laguerre-Gaussian transverse mode intracavity in thecollinear portion of the cavity and (ii) to maintain a circulation ofsuch light in the collinear portion of the cavity. Optionally, the lasersource includes a system configured to cause the laser source to producethe laser output in a pulsed fashion.

In one specific embodiment, the first cavity arm contains acorresponding gain medium element while the second cavity arm is devoidof a corresponding gain medium element. Here, an end-reflector of thesecond cavity arm is partially transparent to light at a pumpwavelength, and the laser source is equipped with a pump laser sourcethat is configured to generate pump light at the pump wavelength andthat is positioned to deliver the pump light through the end reflectoralong an optical axis of the second cavity arm.

In a related implementation, at least one of the following conditions issatisfied: a) at least one of the first and second aims contains anintracavity non-linear frequency conversion element, and b) the outputcoupler is a reflector that either terminates an arm of the laser cavitynetwork or is located within the laser cavity network. In a specificembodiment, alternatively or in addition, each of said first and secondcavity aims may employ respectively-corresponding first and second lasergain media, each of which media a) is designed to amplify light atrespectively-corresponding first and second wavelengths and/or b)includes one of a VECSEL-based gain medium, a solid-state gain medium;and a fiber amplifier. In such specific embodiment, the laser source mayfurther comprise at least one wavelength tuning mechanism juxtaposedwith the cavity network and configured to tune at least one of the firstand second wavelengths independently from another of the first andsecond wavelengths while, at the same time, allowing the first andsecond wavelengths to become equal as a result of such tuning. In suchspecific embodiment, the laser source may contain (in the collinearcavity portion) an optical element defining a spatial fold of a path oflight propagating intracavity and forming, intracavity, first and secondRayleigh regions of first and second light portions of light, where thefirst and second light portions respectively contain the first andsecond wavelengths, and where the first and second Rayleigh regionsoverlap with one another in the spatial fold. (In the latter case, atleast one of the following conditions may be optionally satisfied: a)the optical element is not a reflector that terminates the cavitynetwork and is the at least one output coupler, and b) at least one ofsaid first and second cavity arms contains an intracavity non-linearfrequency conversion element.)

In another related embodiment, at least one of the first and second armscontains a respectively-corresponding gain medium, and the MCE containsan array of mode controller elements that is configured to berepositionable across an axis of at least one of the first and secondarms to cause (i) a formation of an intracavity light having aHermite-Gaussian transverse mode, and (ii) cavity-supported propagationof such light between the MCE and the collinear portion of the cavityduring an operation of the laser source. In this case, alternatively orin addition, the first and second arms may be merged into the collinearportion of the cavity at a beamsplitter, and the laser source isconfigured to maintain and support intracavity light having (a) aGaussian transverse mode between said gain medium and the MCE; (b) insaid Hermite-Gaussian transverse mode between the MCE and the collinearportion of the cavity; and (c) in a Laguerre-Gaussian transverse mode inthe collinear portion of the cavity during the operation of the lasersource.

Embodiments of the invention additionally a method that includes (i)spatially combining first and second light portions (each formingrespectively corresponding first and second standing wave inrespectively corresponding first and second cavity arms of a multi-armlaser cavity network) to form a mixed intracavity beam in which thefirst and second light portions overlap and are co-linearly propagatedthrough a collinear portion of said laser cavity network that is commonfor the first and second cavity arms; and (ii) transmitting at least oneof the first and second light portions through the MCE and the AMC (eachof which is disposed in a corresponding of the first and second cavityarms), while transmitting the mixed beam through a non-linear optical(NLO) element disposed intracavity.

A related embodiment further provides a method including the steps offorming a first intracavity standing optical wave, at a firstwavelength, between and terminated by first and second reflectors of alaser cavity network. (Here, the laser cavity network includes multiplespatially-distinct cavity arms that share a collinear portion of thelaser cavity network. Here, the first intracavity standing optical wavehas a first transverse mode distribution in the collinear portion of thelaser cavity network and a second transverse mode distribution in afirst part of a first arm, where the first part is located outside ofthe collinear portion of the laser cavity network. Here, the first andsecond transverse mode distributions are different from one another.)The method further includes the step of outcoupling a pre-determinedportion of light energy, provided by the first intracavity standingoptical wave, through an outcoupler of the laser cavity network. Here,such predetermined portion of light energy has a second wavelength, thefirst and second wavelengths being different from one another.

In one implementation, the step of forming includes forming the firstintracavity standing optical wave that has a third transverse modedistribution outside of the collinear portion of the laser cavitynetwork and/or at least one of the following conditions (i) and (ii) issatisfied:

(i) the method further comprises:

forming intracavity light at a second wavelength, wherein the secondwavelength is defined as a result of a non-linear frequency conversionof said predetermined portion of light energy inside the laser cavitynetwork, wherein the outcoupling includes outcoupling of light at thesecond wavelength; and

(ii) the outcoupling includes outcoupling of light at the firstwavelength.

(Notably, in a specific case of such implementation, the first andsecond portions of the laser cavity network (in which the firstintracavity standing optical wave has respective first and secondtransverse mode distributions) are configured to be separated from oneanother by a refractive optical element disposed across the firstintracavity standing optical wave. IN another specific case of suchimplementation, at least a part of the second portion of the lasercavity network belongs to only one of the multiple spatially-distinctcavity arms.)

Alternatively or in addition, the method may include at least one of thefollowing steps: (a) amplifying light at said first wavelength containedin the first intracavity standing wave with a first gain mediumcontained in the first cavity arm of the laser cavity network; and (b)forming a second intracavity standing optical wave, at a secondwavelength, between and terminated by a third reflector and the secondreflector of the laser cavity network. Here, a) the second intracavitystanding optical wave has a fourth transverse mode distribution in thecollinear portion of the laser cavity network and a fifth transversemode distribution outside of the collinear portion of the laser cavitynetwork, and/or b) the fourth and fifth transverse mode distributionsare different from one another. Alternatively or in addition, the stepof forming a first intracavity standing optical wave (at a firstwavelength, between and terminated by first and second reflectors of alaser cavity network) may include forming such first intracavitystanding optical wave that has a sixth transverse mode distribution in asecond part of the first arm, such that the second part is locatedoutside of the collinear portion of the laser cavity network. (Here, thesecond part is different from the first part; and the sixth and secondtransverse mode distributions are different from one another.)

In yet another related implementation, the method may additionallyinclude a process of changing at least one of the first and secondtransverse mode distributions as a result of respective intracavityrepositioning of at least one of first and second optical elementsdisposed between the first and second reflectors. (And, in a specificcase of such related implementation, the step of forming the firstintracavity standing optical wave is caused to occur simultaneously withthe changing and the outcoupling.)

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description of Specific Embodiments in conjunctionwith the not-to scale Drawings, of which:

FIG. 1 is a schematic diagram of the VECSEL-based laser systemconfigured to generate, in absence of the MCE, a laser beam in afundamental transverse mode (characterized by a Gaussian distribution oflight irradiance in a plane transverse to the axis of the laser systemcavity);

FIGS. 2A, 2B, 2C, and 2D illustrate transverse irradiance profiles ofthe laser output from the system of FIG. 1 registered with differentcoordinations/orientations of the MCE of the system. FIG. 2A: afundamental Gaussian transverse mode profile; FIG. 2B: the HG₀₁transverse mode profile; FIG. 2C: the HG₁₀ transverse mode profile; andFIG. 2D: the HG₁₁ transverse mode profile;

FIGS. 3A, 3B provide plots illustrating the power and spectralcharacteristics of laser system of FIG. 1 operating in differenttransverse modes (of FIGS. 2A, 2B, 2C, 2D). FIG. 3A: output powercharacteristics for output laser beam having various transverse modedistributions; FIG. 3B: the laser wavelength (spectrum) of the outputlaser beam maintained for all transverse modes and regardless ofswitching from one transverse mode to another;

FIG. 4 is a schematic diagram illustrating the VECSEL-based laser systemconfigured for delivery of light output at a frequency of the secondharmonic of light generated at a fundamental frequency and amplifiedintracavity, in different (tunable) transverse modes;

FIGS. 5A, 5B, 5C illustrate plots representing simulation results as afunction of the scaled phase-mismatch for an HG₀₁ beam output at afundamental frequency (first harmonic). FIG. 5A: power at asecond-harmonic (SH) frequency relative to power at a first-harmonic(FH) frequency; FIG. 5B: intensity profile of the light output at a SHfrequency; and FIG. 5C: the effective mode index;

FIGS. 6A, 6B, 6C provide plots representing simulation results (similarto those of FIGS. 5A, 5B and 5C) as a function of the separationdistance between the non-linear optical crystal and the planar reflectorof the system of FIG. 4 for a fundamental harmonic (fundamentalfrequency) HG₀₁ beam output. FIG. 6A: power at SH relative to the powerat FH; FIG. 6B: intensity profile of the light output at a SH-frequency;and FIG. 6C: the effective mode index;

FIGS. 7A through 7D present captured images of the green light outputhigh-order transverse mode beams, generated by the system of FIG. 4,with HG₀₁-like (FIG. 7A), HG₀₂ (FIG. 7b ), HG₁₁like (FIG. 7C), and Hg₂₂(FIG. 7D) transverse intensity distribution profiles (transverse modes);

FIG. 8A: plots representing output power characteristics for green (SH)spatially-structured beams with various HG and HG-like transverseprofiles, generated with the system of FIG. 4; FIG. 8B illustrates thatthe lasing wavelength for SH radiation output was meticulouslymaintained for all different transverse modes and regardless ofswitching from one transverse mode to another;

FIGS. 9A, 9B, 9C, 9D present the results of simulation corresponding toFIGS. 7A, 7B, 7C, 7D respectively. FIG. 9A: HG₀₁like spatialdistribution; FIG. 9B: HG₀₂ distribution; FIG. 9C: HG₁₁likedistribution; and FIG. 9D: Hg₂₂ distribution;

FIG. 10 schematically illustrates an embodiment employing the linearT-cavity VECSEL laser system with an external (with respect to thecavity) astigmatic mode converter;

FIG. 11 illustrates empirically measured lasing spectra of the lightoutput produced by the two-arm laser system of FIG. 10, showing twopeaks at ˜972 nm and ˜1072 nm (produced, respectively, by theoperationally-independent cavities containing chips 1100 and 1200,respectively);

FIGS. 12A, 12B are plots illustrating independently-tunable spectra ofthe components of the output 1500 (of the system of FIG. 10) obtained byindependent operation of the BFs in two arms of the system's cavitynetwork. FIG. 12A: tuning of the lasing wavelength of the chip 1100;FIG. 12B: tuning of the lasing wavelength of the chip 1200;

FIG. 13: the output power for outputs at various transverse modes,generated by the T-cavity VECSEL system of FIG. 10;

FIGS. 14A, 14B, 14C, 14D: Beam profiles of various higher-ordertransverse mode generated with the embodiment 1000. FIG. 14A: ˜970 nmchip 1100; HG₀₁ and LG₀₁ modes; FIG. 14B: ˜1070 nm chip 1200, HG₀₁ andLG₀₁ modes; FIG. 14C: ˜1070 nm chip 1200 m HG₁₁ and LG₁₀ modes; and FIG.14D: overlapped (1722) and misaligned (1724) LG₀₁ modes from ˜970 nm and˜1070 nm chips 1100, 1200.

FIG. 15 is a schematic diagram of a related embodiment configured,according to the idea of the invention, as a two-cavity-arm embodimentof the laser cavity network;

FIG. 16 is a schematic of a folded T-cavity VECSEL-based laser-cavitynetwork with one passive cavity arm, configured to allow for controllingthe transverse distribution of the laser light output (the LG modecontrol) and field enhancement. The combined intracavity beams,supported in individual cavity arms, are coaxially overlapped in thecommonly-shared portion of the cavity to generate the laser output withthe same transverse properties as those of the light distributionpresent in the passive cavity arm;

FIG. 17 is a schematic diagram of a folded T-cavity VECSEL-basedlaser-cavity network used for type II SFG with intracavity MCEconfigured as a wire to obtain an HG₀₁ mode distribution of light in thecavity arm containing the MCE. The external (positioned outside of thelaser-cavity network) astigmatic mode converter is configured totransform the HG₀₁ transverse distribution of the laser output to anLG₀₁ transverse distribution in light that has traversed the externalmode converter;

FIG. 18 is a diagram schematically illustrating a related embodiment ofthe collinear T-cavity VECSEL device structured according to the idea ofthe invention for generation of the laser light output havingchangeable/tunable transverse mode distribution with the spectrumcorresponding to the type-II sum frequency generation or differencefrequency generation based on the frequencies of light amplified indifferent arms of the cavity network;

FIG. 19 is a diagram illustrating a related embodiment with a T-cavityVECSEL structure configured to generate the UV light (optionally, inaddition to the generation of the IR and/or visible light) in differentand spatially-changeable transverse modes.

Generally, the sizes and relative scales of elements in Drawings may beset to be different from actual ones to appropriately facilitatesimplicity, clarity, and understanding of the Drawings. For the samereason, not all elements present in one Drawing may necessarily be shownin another.

DETAILED DESCRIPTION

Implementations of the present invention provide a novel on-demand lasersource configured to generate spectrally-tunable laser beam(s) in(generally variable during the frequency-tuning process) LG modes. Theidea of the present invention stems from the realization thatintracavity generation and nonlinear mixing of coaxial Gaussian (G) andHermite-Gaussian (HG) modes in a multi-color, T-cavity laser device(such as one, built around a Vertical External Cavity Surface EmittingLaser, or VECSEL, for example) permits efficient HG and/or LG mode(s)generation.

The Laguerre-Gaussian (LG) beams—that is, laser beams having atransverse distribution of optical power (or transverse modes) describedby LG functions—can probably boast the most prolific realization inpractice. The beams with LG transverse modes have been used to enhancethe information capacity of both classical and quantum communicationchannels by employing many orthogonal optical angular momentum (OAM)modes as opposed to two polarization modes. Other key applications ofthe LG modes derives from the ability of the light OAM to be transferredto and rotate particles such as atoms and nanoparticles, driving opticalmicro-machines, and excitation of matter-wave vortices in cold atomicgases. Still other applications arise in quantum entanglement of twistedphoton states and quantum information processing, spiral phase contrastmicroscopy, and astrophysics and space physics studies. Overall, it israther undisputed that the ability to produce beams having diverse LGmodes will continue to have significant impact in the fields of quantumcommunication, where LG beams are used to increase the informationcapacity, laser trapping where the laser can provide novel trappingpotentials, and laser writing of optical structures such as extendedwaveguides using Bessel beams. It is of significant interest to have asource of novel laser beams with high power and over a large range ofwavelengths for the variety of applications.

Despite familiarity with LG modes in applied and basic science, nounified optical source configured to generate laser beam(s) in suchmodes at high-power levels while permitting for spectral tunability ofthe laser beam(s) exists up to-date: a few known attempts to generate LGlaser beams produced results limited in power and/or operatingwavelength range. One means of generating LG beams related art can referto made use of a spatial light modulator (SLM) device (such as one builtaround a MEMS structure), which is known to operate at wavelength(s)falling within the range of about 400 -1064 nm and limited to being ableto handle a few Watts of power before damage of the individualreflectors of the SLM device sets in. While higher power LG beams couldarguably be realized using fused silica phase-masks, a different phasemask is necessary for each realization of the OAM. Likewise, althoughspecific lasers can be coaxed to operate on LG modes, such a solutionimposes a limit on the spectral tenability of the produced LG-laserbeams and requires a multiplicity of the laser sources that inevitablydrives up the costs of the overall system.

VECSEL-based laser systems so far have been developed to provide highoutput powers over a wide range of fundamental lasing wavelengths (670nm-2.4 μm) as a result of semiconductor heterostructure bandgapengineering. In particular, the well-developed VECSEL heterostructuresbased on GaAs/InGaAs semiconductors are considered to be preferred forgenerating high power continuous wave (cw) outputs in the 900 nm to 1200nm range. The free-space external cavity of VECSELs combined withoptical pumping allows for excellent mode quality, while the access tothe high intracavity circulating power also allows for efficient secondharmonic generation (SHG) providing tunability via the second-harmonicin addition to the fundamental harmonic, and single frequency operationcan be obtained if intracavity etalons or birefringent filters are used.

In aiming to produce a high-power wavelength tunable beam, however, theVECSEL-based technology thus far has been exclusively focusing on doingso with a Gaussian transverse profile. On the other hand, higher-ordermodes other than the Gaussian are currently of great fundamental andtechnological interest (including Hermite-Gaussian (HG),Laguerre-Gaussian (LG), Bessel-Gaussian (BG), Airy, and Helmholtz beams,to mention just a few). While some attention has been paid to lasersystems for generating these higher-order modes (including LG modes froma microlaser, optically pumped solid state lasers, andspatially-structured vertical cavity surface emitting lasers with modeshaping with integrated optical elements), none of the consideredpossibilities offered the potential for such high-order spatial modesbeams to combine high power and spectral tunability. In addition, noneof the considered laser-structure options is necessarily as well adaptedto intracavity SHG or other forms of wave mixing as VECSELs are.

Similarly, to date the research on VECSELs was mainly focused onpower-scaling or short-pulse generation at various wavelengths. However,the use of VECSEL-based laser systems for generation of higher-order LGmodes carrying orbital angular momentum (OAM) beams has not beenexplored.

The implementations of this invention fill the gaps in the ability ofcurrently-available technology to generate these sought-after lightbeams. In particular:

1) The unaddressed need of enabling a laser device to generate light ata chosen wavelength or, simultaneously at a multiplicity of wavelengths(whether in a spectrally-stationary or spectrally-tunable fashion)while, at the same time, being able to control and vary the spatialdistribution of light output from the laser device in a plane transverseto the optical axis (referred to herein as a “transverse mode” of alaser beam) is solved by devising a laser-cavity network that containsmultiple spatially-distinct laser cavity arms which (i) share, amongthemselves, at least one portion of the cavity network, and (ii) atleast one of which contains, intracavity, a mode-control element (MCE)and/or an astigmatic mode converter (AMC), and, optionally, (iii) atleast one of which contains a non-linear frequency conversion elementintracavity.

For the purposes of this disclosure, an MCE is defined as an opticalsystem configured to make a laser cavity (with which the MCE isjuxtaposed) support lasing at a predetermined transverse mode of lightthat is defined by the configuration of the MCE. An MCE may beconfigured an optical element or system causing loss of light, whichloss has a predetermined distribution in a plane transverse to the axisof the laser cavity. Alternatively, the MCE may include an optical gainelement causing the amplification of light intracavity in accord with aspecific predetermined distribution of gain across the beam amplifiedintracavity. An AMC is an optical system causing a re-distribution orspatial restructuring of first light, having first transversedistribution or mode, into second light having second transversedistribution or more upon the transmission of the first lighttherethrough.

Here, the control of the transverse mode of the laser output iseffectuated at at least one of the wavelengths present in the laseroutput, and at least one cavity arm of the overall cavity network may bepassive (and not contain a gain medium intracavity). It is appreciatedthat, in general, the nonlinear frequency conversion may be requiredonly if the wavelength of the LG modes needs to be changed through somenon-linear conversion mechanism such as, for example, second harmonicgeneration (SHG), sum frequency generation (SFG) or difference frequencygeneration (DFG). Otherwise, without a nonlinear optical elementdisposed intracavity the co-axial superpositions of the LG modes atfundamental wavelengths is still created in an embodiment of theinvention.

The MCE(s) and AMC—taken separate or as a combination—areinterchangeably referred to herein as a transverse mode converter system(TMCS). Since at least one portion of the cavity network is being sharedbetween or among at least two cavities, intracavity light portionspropagating spatially independently in different cavity arms of thecavity network are propagating co-linearly in such shared portion orregion of the cavity network. The shared portion of the devised cavitynetwork may be referred to, therefore, as a “co-linear portion of thecavity network” or “collinear cavity portion”, for short.

2) The unaddressed need of enabling a laser device to simultaneouslygenerate light at two or more wavelengths that are independently tunablewithout a practical limit of how small a spectral separation betweensuch wavelengths can be made while, at the same time, being able tocontrol the transverse mode of the laser output at at least one of thewavelengths present in the laser output is solved by devising alaser-cavity network that contains such multiple spatially-distinctlaser cavity arms which (i) not only share among themselves at least oneportion of the cavity network, but are also defined by optical elementspreventing the intracavity amplification of light at two of thesewavelengths via amplification processes occurring in the same laser gainmedium, and at least one of which (ii) contains, intracavity, a TMCS.Here, an open cavity network of a laser device of the invention isstructured to support a first process of laser light amplification atthe first wavelength and a second process of laser light amplificationat the second wavelength, which processes (i) do not share the same gainbandwidth but respectively correspond to different gain curves, and (ii)propagate along spatially different intracavity optical paths that sharea common optical path portion along the direction of light propagation,while the open cavity network includes a TMCS intracavity. In sucharrangement, each of the cavity arms of the overall cavity network isactive in that it includes a specifically-dedicated andrespectively-corresponding gain medium (such as, in one non-limitingexample, a VECSEL gain medium). Here, the control of the transverse modeof the laser output is effectuated at at least one of the wavelengthspresent in the laser output. Moreover, in stark contradistinction withrelated at, in such configuration the control of transverse laser modescan be effectuated simultaneously and/or independently in two or more ofthe cavity arms of the overall cavity network.

Accordingly, below are discussed but several examples of a laser source,structured according the idea of the invention and based on sum- ordifference-frequency generation in a T-cavity VECSEL, for bothcontinuous-wave (CW) and pulsed operation.

The specific case of T-cavity VECSEL configuration facilitates theprocess of controlling the transverse mode of laser light distributionindependently in multiple laser cavity arms: For example, one arm may beoperating on the Gaussian or HG₀₀ mode and the other on the HG₀₁higher-order mode. Intracavity nonlinear frequency generation may thenbe used to produce a new high power novel laser beam, depending on thetransverse modes employed. The frequency of the novel laser beam can becontrolled via the resonant frequency in each arm and the choice of sum-or difference-frequency generation.

NON-LIMITING EXAMPLES OF EMBODIMENTS Example 1

This implementation provides demonstrates that a linear-cavity VECSELstructure, containing an MCE intracavity, is enabled to controloscillation of the cavity field in a selected higher-order HG beam atthe fundamental wavelength.

In the experiments reported here, MOCVD was used to grow a VECSELheterostructure designed to emit at about 1070 nm. The active regionincluded 12 compressively-strained 8-nm-thick InGaAs quantum wells (QWs)with GaAs-pump-absorbing barriers and a layer of GaAsP between eachneighboring QWs for strain compensation. On top of the multi-quantumwell (MQW) unit the 25 pairs of alternating AlGaAs/AlAs layers weregrown to operate as a high reflectivity (˜99.9%) distributed Braggreflector (DBR) at the emission wavelength. The specific thickness andcomposition of the heterostructure layers were judiciously chosen toachieve resonant periodic gain (RPG) such that each QW is positioned atthe antinodes of the standing optical wave formed in the laser cavity.Thermal management of operation of the laser structure was accomplishedby solder-bonding of a VECSEL chip to a CVD diamond element. Oncebonded, selective chemical wet etching was used to remove the GaAssubstrate. The surface of the chip was antireflection (AR) coated at thepump wavelength of about 808 nm, and the overall device was mounted andclamped to a water-cooled copper heat-sink system for temperaturecontrol.

As shown in the embodiment 100 of FIG. 1, a standard linear laser cavityconfiguration was used. The VECSEL chip 110 together with a 30 cm radiusof curvature (RoC) reflector 114 (configured to operate as a 97%reflective output coupler or outcoupler) formed an approximately 25 cmlong resonant cavity. The fiber-coupled 808 nm diode pump beam 118 wasrefocused with the lens 120 onto the surface of the VECSEL chip 110 to aspot with diameter of about 400 μm, which was slightly larger than the˜380 μm fundamental transverse mode size defined by the cavity lengthand the reflective outcoupler's RoC. The copper heat sink 122 wasmaintained at a temperature of 15° C. The MCE 126 was placed inproximity of the output coupler 114 intracavity, which allowed foroperation on higher-order spatial modes as opposed to the Gaussian mode.In one specific case, the MCE 126 was structured as a fused-silicatransparent substrate selectively patterned to provide a lower lossfigure for a targeted HG mode distribution of the laser radiationintracavity as compared to all other modes (which resulted, during theoperation of the embodiment 100, in the highest gain for the targeted HGmode).

During the operation of the system 100, the position and/or orientationof the MCE 126 was adjusted to achieve different Hermite-Gaussian modeintensity profiles, and the output powers and beam profiles of the laseroutput 130 were measured. In order to characterize the transverse beamstructure, a DataRay BeamMap2 scanning slit beam profiler was used. Toobtain an image of the lasing mode, the beam was refocused into theprofiler sensor to a size of tens of microns in diameter. FIGS. 2A, 2B2C, and 2D illustrate the fundamental mode as well as other recordedshapes of an output beam 130, and specifically the HG₀₀, the HG₀₁, theHG₁₀ and the HG₁₁ transverse intensity profiles.

FIG. 3A presents the output power characteristics for the fundamentalGaussian lasing as well as those for HG₀₁, HG₁₀ and HG₁₁ transversemodes of operation of the system 100. The Gaussian mode produced thehighest output power exceeding 4 W. The output power in the HG₀₁ modebeam was about 3.8 W, whereas both when the output 130 contained theHG₁₀ mode and when the output contained the HG₁₁ the peak output powerwas about 2.5 W. This result is readily understood with recognition thatthe MCE 126 introduced a small amount of loss and that the pumped gainarea at the chip 110 (optimized for the Gaussian mode) provided lessgain to the higher-order transverse modes (each of which ischaracterized by a larger effective area than the Gaussian mode).Moreover, the HG₁₀ mode experienced less gain than the HG₀₁ mode byvirtue of the fact that the pump beam was incident onto the chip 110 atan angle, thereby leading to an elliptically shaped spatial gainprofile: this gain profile asymmetry is responsible for the lower outputpowers for the HG₁₀ and HG₁₁ modes in FIG. 3A. Since the pump spot arearemained fixed for all modes, the generation of the HG₁₁ mode occurredat a lower level of output power as compared to the other modes. FIG. 3Billustrates that the output wavelength was maintained in all cases atabout 1063 nm. The measured output powers and wavelength were provenstable within the 1 Å resolution limits of the utilized optical spectrumanalyzer (OSA) and the resolution of the power meter.

Example 2

Spatially-Structured Beam via SHG in a V-cavity VCSEL.

Here, as shown in FIG. 4, an embodiment 400 employed a common V-foldedlaser cavity configuration. This cavity type, compared to linear cavityof FIG. 1, is advantageous in the case of nonlinear conversion becauseit allows controlling the size of the transverse mode in both the gainmedium and the nonlinear crystal. The spherical concave reflector 414with a radius of curvature of 10 cm served as a folding mirror to form afolded portion of the cavity (“fold”) 418 folded with respect to thelinear portion of the cavity 422, while the VECSEL chip 110 and the flatend reflector 426 defined and enclosed the overall resonant cavity. ForSHG operation, an LBO crystal 430 (3×3×15 mm³; both facets AR coated forboth 1075 nm and 537 nm), cut for type I angular phase-matchingcondition with angles θ=90° and ϕ=11°, was inserted into the shorter arm(fold 418) of the laser resonator a variable distance d away from theflat mirror 436. Both reflectors 414, 426 carried an HR coating for highreflection of light at 1070 nm fundamental wavelength, thereby ensuringhigh lasing circulating power at a fundamental wavelength, while theflat mirror 426 was additionally HR coated for a second wavelength ofabout 535 nm. This allowed all of the green light to be extractedthrough the folding minor that had a low reflectivity (<10%) coating at535 nm.

The same optical pumping arrangement as that of FIG. 1, with a pump spotof ˜400 μm in diameter on the chip surface, was utilized. In a fashionsimilar to that of FIG. 1, the separation distances between the VECSELchip 110 and the minors 414, 426 were chosen to ensure lasing in afundamental Gaussian transverse mode while mode matching to the size ofthe optical pump 118 spot at the chip 110 and maintaining theappropriate focusing into the nonlinear crystal. (In one case, thedistance from the chip 110 to the curved minor 414 was ˜21 cm, and thedistance from the curved minor 414 to the flat minor 425 was about 6cm). Based on these cavity dimensions, the calculated fundamental modediameter was equal to about 100 μm at the flat minor 425 and about 380μm at the surface of the chip 110. The same MCE 126 was then insertedinto the cavity to control which HG mode the fundamental frequency beamof light operated on. In addition, a 3 mm thick birefringent filter (BF,434) was employed for wavelength stabilization and for maintaining anarrow linewidth of lasing light.

Theoretical model and simulations. The simulations were performed in theundepleted pump beam approximation. Following the notation used by R. W.Boyd in (Nonlinear Optics Academic, Amsterdam, 2008, 3^(rd) Ed., Chap.2), the propagation of the second-harmonic field in the LBO crystal 430is described by the paraxial wave equation

$\begin{matrix}{{\pm ( {\frac{\partial A_{2}}{\partial z} + {\delta \frac{\partial A_{2}}{\partial x}}} )} = {{\frac{1}{2k_{2}}{\nabla_{T}^{2}A_{2}}} + {\frac{i\; \omega_{2}^{2}d_{eff}}{k_{2}c^{2}}A_{1}^{2}e^{i\; \Delta \; {kz}}}}} & (1)\end{matrix}$

where A_(1,2) are the slowly varying envelopes of the fundamental andsecond-harmonic fields that are propagating dominantly along the z-axis;the ± signs correspond to propagation in the forward and backwarddirections in the LBO crystal. Diffraction was accounted for via thetransverse Laplacian term, as was the walk-off with angle δ along thex-axis. Here d_(eff)≃1 pm/V is the effective nonlinear coefficient, andΔk is the wavevector mismatch. The fundamental field obeys a similarequation with no walk-off (θ=90° is maintained) or nonlinearity, and thesubscript 2 is replaced by 1. Eq. (1) assumes that the second-harmonicfield is polarized along the x-axis, and so is the direction of thewalk-off

Both in the experiment and the simulations there are two controlparameters—the distance d and the wavevector mismatch Δk. Based on theestimated spot size of light fundamental frequency w₀=50 μm at the flatminor 426, and the transverse mode selected by the MCE 1256 foroperation at the fundamental harmonic (FH) by the MCE 125, for the HGtransverse mode with mode indices (n, m) the fundamental frequencylight-field at the flat minor 426 is set as

$\begin{matrix}{{{A_{1}( {x,y} )} = {A_{0}{H_{n}( \frac{x}{\sqrt{2}w_{0}} )}{H_{m}( \frac{y}{\sqrt{2}w_{0}} )}e^{{- {({x^{2} + y^{2}})}}/w_{0}^{2}}}},} & (2)\end{matrix}$

where the parameter A₀ controls the power at the FH. Then, given thedistance d, the FH light field was determined at the input facet 430A ofthe LBO crystal 430 using linear propagation, along with thecorresponding spot size w_(in) at the input facet 430A. The FH and SHfields were then propagated over the length L of the crystal using theBeam Propagation Method known in the art, from the input facet 430A tothe facet nearest the flat mirror 426. Both the FH and SH fields werethen propagated free-space style (with the use of a standard FourierOptics methods) over the distance 2 d back to the crystal 430, includingthe phase difference arising from the dispersion of air. Finally, the FHand the SH fields were propagated in reverse direction—through the LBOcrystal 430—to determine the profile of the output SH field 438.

In the following, the walk-off effect is neglected. The second-harmonicpolarization P₂ is in general driven by the square of the fundamentalfield P₂ ∝A₁ ². Now if the fundamental field is taken as aone-dimensional Hermite-Gaussian for illustration, then the polarizationfor the second-harmonic will vary as

$\begin{matrix}{{P_{2}(x)} \propto {{H_{m}^{2}( \frac{x}{\sqrt{2}w_{0}} )}e^{{- 2}{x^{2}/w_{0}^{2}}}} \propto {\sum_{k = {- \infty}}^{\infty}{a_{k}{H_{k}( \frac{x}{w_{0}} )}e^{{- 2}{x^{2}/w_{0}^{2}}}}}} & (3)\end{matrix}$

That is, the SH field will be driven by a polarization that is asuperposition of HG beams with amplitudes a_(k) and based on the spotsize w₀/√{square root over (2)}, and therefore—depending onphase-matching—the output at the SH can be presented by a variety ofspatial distributions.

Turning now to the simulation results, FIGS. 5A, 5B, 5C present resultsas a function of the scaled phase-mismatch σ=Δkw₀ ² for afundamental-harmonic HG₀₁ beam, so (n, m)=(0,1), and d=0 (that is, theLBO crystal 430 is disposed up against the flat mirror 426). The scaledphase-mismatch can be tuned by slightly varying the angle of the crystal430. FIG. 5A shows the generated SH power relative to the FH power,while FIG. 5B shows the calculated SH output intensity profile |A₂(0,y)|² as a function of the scaled coordinate y/w_(in); w_(in) being thespot size of light at the FH at the input facet 430A. The SH profileremains largely Gaussian along the x-axis in this example. In FIG. 5B,the peak intensity is normalized to unity in each case so that theprofiles for different a can be compared. A person of skill in the artwill readily recognize that, as the phase-mismatch is varied, thespatial structure (spatial distribution) of the generated SH lightchanges drastically along the y-axis, from HG₀₂ at σ=0 to Gaussian-likeat σ=−2. To give a qualitative measure of the mode content, in FIG. 5Cthe effective mode index m_(eff) is plotted. This is obtained bycalculating the numerical value of the beam quality M² (see A. E.Siegman, “How to (maybe) measure laser beam quality.” In Diode PumpedSolid State Lasers: Applications and Issues, p. MQ1. Optical Society ofAmerica, 1998), evaluated along the y-axis for the SH field, and usingthe relation m_(eff)=(M²−1)/2, which is known to be exact for HG modes.It can be observed, therefore, that phase-matching can be used as aparameter to control the spatial distribution of the generated SH light.FIGS. 5A, 5B, 5C also attest that the peak SH power need not coincidewith where integer values of m_(eff) arise. Accordingly, there is atradeoff between trying to maximize power and realizing a given spatialfield structure.

FIGS. 6A, 6B, 6C show the same results as those of FIGS. 5A, 5B, 5Cexcept for σ=0 and as a function of d (the separation distance betweenthe non-linear crystal medium and the nearest reflector. FIG. 6Bdemonstrates that d can be used to control the spatial profile. Forexample, around d=25 μm one finds m_(eff)≃2. FIG. 6A suggests lowconversion efficiency at that point, but this can be countered byvarying the phase-matching parameter σ. In this example, peak power wasobtained for σ=−1. Notably, this coincides with what was later doneexperimentally, that is, for a given value of d, the crystal angle canbe adjusted to strike a balance between power and the spatialdistribution of light generated at the SH. Based on the results of thesimulation, it can be found that for a HG_(0m) FH beam the effectivemode index m_(eff) at the SH can range between zero and 2m as thephase-matching is changed, leading to a rich variety of spatial modes.This can be further extended to the case of a HG_(nm) FH beam (notingthat the effective mode index evaluated along each direction can varyfrom zero to 2n, and zero to 2m).

Experimental results. This section presents the results of measurementof properties of spatially-structured SH beams, that were generated withthe use of a laser system 400 by utilizing the higher-order HG modesexcited at the FH wavelength (˜1070 nm) in the V-cavity equipped withthe MCE 126. As a reference, initially the Gaussian mode SHG output wasmeasured for a Gaussian FH intracavity light in absence of the MCE 126in the cavity. Next, the MCE 126 was introduced to create a circulatingHG₀₁ FH mode, which was then converted at the LBO crystal 430 into thegreen output at 438. The spatial structure of the green beam 438depended on the fine-tuning of the phase-matching angle as well as therelative position of the crystal 430 and the mirror 426 (i.e., on thebeam waist position) as predicted by the simulations discussed above.

FIGS. 7A, 7B, 7C, 7D show the empirically-measured green (SH) beams fora) d=25 mm , yielding a HG₀₁ beam, and for b) d=0 mm, yielding a HG₀₂beam. As discussed above, although FIG. 7A shows an intensity profilesimilar to that of a HG₀₁ beam, it cannot have the associated phaseprofile, and for this reason the terminology “HG₀₁-like” beam is used.This terminology has practical relevance as many applications (such asoptical trapping, for example) rely on the intensity profile of the HGmodes and are not dependent on the associated phase. Similarly, FIGS. 7Athrough 7D also show the measured profiles for a HG₁₁ fundamentalharmonic for c) d=25 mm, yielding a HG₁₁-like beam, and for b) d=0 mm,yielding a HG₂₂ beam. These results demonstrate that the VECSELstructure with intracavity nonlinear frequency conversion element isconfigured to generate a rich variety of spatially-structured SH beams.

The output power at SH was also characterized for each of the structuredbeams and the results are compared in FIG. 8A. The green (SH) output forthe HG₀₀ fundamental mode had the highest power of about 3.1 W. For thestructured beams, the SH power decreases for the same reasons that werediscussed in reference to FIGS. 3A, 3B, in addition to there being alower nonlinear conversion efficiency. The HG₀₁like and HG₀₂ modesreached maximum power of about 1 W and 2.6 W, respectively, while theHG₁₁-like and Hg₂₂ modes highest powers were registered to be about 0.21W and about 0.6 W, respectively. The significant power drop betweenthese two pairs of outputs is a result of the LBO crystal's placement inthe laser cavity. While for HG₀₂ and Hg₂₂ beams, the crystal 430 wasplaced near the flat mirror 426, for the HG₀₁like and HG11-like beamsthe crystal 430 was placed 25 mm away from the flat mirror 426. In thelatter case, the nonlinear conversion occurred for a highly expandedbeam, w_(in)>>w₀, leading to lowered output powers. Finally, FIG. 8Bpresents the spectrum for the SH light that peaks around 530 nm and hasa linewidth of about 0.1 nm (which was maintained throughout themeasurements)

FIGS. 9A, 9B, 9C, and 9D) show the simulations results corresponding toFIGS. 7A, 7B, 7C, and 7D with the following parameters: for plots (a, c)d=25 mm and σ=−1, and for plots (b, d) d=0 mm and σ=0. A goodqualitative agreement between the theoretical prediction and theempirical results is evident. Overall, a person of skill in the art willreadily appreciate that Examples 1 and 2 convincingly prove thatVECSEL-based laser system can be configured to generate high-powerspatially structured light output at the fundamental wavelength with theuse of an MCE, whereby Hermite-Gaussian modes can be selected at adiscretion of the user of the system. Using this approach in conjunctionwith intracavity SHG, the spatial structure of the SH light output canbe manipulated via phase-matching along with the optional spatialrepositioning of the nonlinear crystal inside the laser cavity.

The following Examples 3 and 4 disclose the designs of and experimentalresults obtained with multi-chip T-cavity VECSEL-based laser systemsutilized for multi-color collinear generation of Hermite-Gaussian andLaguerre-Gaussian transverse modes. Here, (i) a combination of anintracavity MCE(s) and an external (with respect to the cavity)astigmatic mode converter (AMC), such combination forming a TMCS ofExample 3; and (ii) a combination of the intracavity MCE(s) and theintracavity AMC(s) were used to achieve high-power LG modes, suchcombination forming the TMCS of Example 4. By incorporating intracavityBFs in each arm of the T-cavity in Example 3, wide wavelength tuningrange (in excess of 14 nm) for each mode were demonstrated, while theoutput power exceeding 1 5 W was measured for all the modes.

The multi-color T-cavity VECSEL-based system is an advantageousmodification of conventional VECSEL cavities, simultaneously allowingfor the generation of high-power multi-color coaxial emission and abroad-range of wavelength generation through intracavity nonlinear sumor difference frequency conversion. See, for example, U.S. Pat. No.9,231,373, the entire disclosure of which is incorporated herein byreference. The Example 3 below discloses the first demonstration ofhigher-order HG and LG modes generated in a two-color T-cavity VECSELgeometry. By combining intracavity mode-control elements (MCEs) withexternal astigmatic mode converters (AMC), a wide range of HG and LGmode pairs were demonstrated. An MCE disposed into a resonatorintroduces a localized loss in the transverse field, thus allowing theskilled artisan to obtain a variety of HG modes at both generatedwavelengths. With the addition of an AMC, various LG modes for eachwavelength are generated, while maintaining the high output power andwavelength tuning properties of each cavity.

Example 3

The separate VECSEL chips used in the experimental setup schematicallydepicted in FIG. 10 were fabricated from two different wafers withstrain-compensated InGaAs/GaAs/GaAsP multi-quantum-well (MQW)heterostructures designed for emission at about 970 nm and at about 1070nm. An MCVD process was utilized to grow the wafer in a“bottom-emitting” manner, such that the active region precedes a DBR ona GaAs substrate. To maximize the gain, in both chip structures thecompositions and thicknesses of the gain regions were carefully chosensuch that each QW were positioned at the antinode of the resonatorstanding wave—a design referred to herein as a resonant periodic gain.While both structures have active regions consisting of 12 compressivelystrained 8 nm thick InGaAs QWs with pump absorbing GaAs barriers andGaAsP layer between each QW for strain compensation purposes, thesemiconductor compositions slightly varied between the wafers used for970 nm and 1070 nm chips. Similarly, the designs of the DBR stacksincluding 25 pairs of alternating AlGaAs/AlAs were adjusted to achievehigh reflectivity (˜99.9%) of light at each wavelength.

To facilitate optimal thermal management to achieve efficient lasing,pairs of a CVD deposition diamond and a corresponding coated VECSEL chipwere coated with Ti/Au layers and then indium solder bonded together.The removal of GaAs substrate with selective chemical wet etchingguarantees an optically flat surface of a finished bonded chip. Upon thefabrication completion, the chips were mounted and clamped towater-cooled copper heat sinks and maintained at a temperature of 15° C.

The T-cavity setup 1000, along with the AMC used for the second part ofthe experiment, is shown in FIG. 10. (For the first part of theexperiment, in which HG modes were generated, the AMC were notutilized.)

The first arm 1100 of the embodiment 1000 (which contains a first VECSELgain chip 1140) supports the lasing at wavelength λ₁ (for example, 970nm), while the second arm 1200 (which contains a second VECSEL ship1240) supports the lasing at wavelength λ₂ (for example, 1070 nm). Abirefringent filter (BF) 1260 in the arm 1100 is oriented at aBrewster's angle such that light 1280 circulating along the arm 1100 isp-polarized (TM lasing mode). A BF 1360 of the arm 1200, on the otherhand, is oriented at a Brewster's angle in such a way that the plane ofincidence for light 1380 circulating in the arm 1200 definess-polarization (TE lasing mode). Each of the (3 mm thick, in oneimplementation) BFs 1260, 1360 not only selects a linear polarization ofthe corresponding portion of intracavity light, but also facilitates thewavelength tuning operation of the device 1000 in the corresponding arms1100, 1200. It is intended that related embodiments of the invention canutilize different spectrum-tuning mechanisms such as, for example, aspatially-repositionable intracavity diffraction grating or prism, anoptical etalon, or an external cavity, or a temperature-tuningmechanism, to name just a few. According to one embodiment of theinvention, each of the BFs includes quartz plates of optical qualityoriented, with respect to planes of incidence of the respectiveintracavity fields 1280, 1380 such as to prevent optical couplingbetween these fields. The polarization selectivity in the cavity aims ofthe embodiment 1000 can be achieved, for example, by placing the BF's atBrewster angle. Therefore, generally an embodiment of the inventionincludes at least one wavelength tuning mechanism disposed within thecavity network and configured to tune a wavelength of light in at leastone of the first and second cavity aims of the cavity network.

The optical field present in each of the regions 1100, 1200 isoperationally independent from any other optical field in any otherportion of the overall cavity of the device 1000 and does not share thegain medium with any other optical field. Accordingly, auxiliaryelements optionally present in regions 1100, 1200 interact only with thelight fields at λ₁, λ₂, respectively. The common, co-linear region 1400of the cavity is the cavity portion between the beam splitter 1440 andthe output coupler 1480, which were shared by the optical fields 1280,1380. Here, a dichroic mirror 1440 which was AR coated at λ₁˜970 nm andhigh reflectivity (HR) coated at λ₂˜1070 nm for an angle of incidence ofabout 22.5°, which allows a person of skill in the art to havecompletely independent laser cavity portions 1100, 1200 sharing the sameoutput coupler mirror 1480 and producing collinear output beams. Thereflector 144 ensured that p-polarized light 128 propagated unabatedwhile the s-polarized light was reflected in a substantially transversedirection (in one implementation, at about 90 degrees, in anotherimplementation—at about 120 degrees) with respect to the z-axis. In thecavity region 1400, the two individual optical fields at wavelengths λ₁and λ₂ overlap spatially to form an overall optical intracavity fieldand the corresponding output 1500 characterized by the two wavelengthscorresponding to two orthogonal linear polarizations.

Each of the arms 1100 and 1200 were complemented, in part of theexperiment, with a respectfully-corresponding MCEs 1520, 1530 (shown indashed lines) such that each of the two independent resonators orcavities (the first one defined between the chip 1140 and the mirror1480, and the second one defined between the ship 1240 and the mirror1480) contained dedicated intracavity BF and MCE. While the BFs allowfor independent wavelength tuning and polarization control based on BForientation, the presence of MCEs enables laser operation athigher-order transverse modes (in an aim with an MCE). In oneimplementation, each of the MCEs was made of a fused-silica transparentsubstrate, which was selectively etched to provide a pattern generatingthe least loss for a targeted HG mode (while increasing losses fornon-desired HG modes). Thus, a two-color individually tunable output1500 with orthogonally polarized beams was generated, though if requiredthe light output components corresponding to different cavity aims 1100,1200 could be made of the same polarization. Since the MCEs were placedbetween the chips 1140, 1240 and the beamsplitter 1440, eachoperationally-independent cavity was configured, generally, to produce adifferent transverse mode. The light outputs of two 808 nm pump laserdiodes (not shown) were fiber-coupled coupled and focused to formapproximately 500 μm diameter spots on each of the VECSEL chips 1100,1200 to provide optical pumping. In a specific embodiment, the sharedoutput coupler mirror 1480 had a radius of curvature of 25 cm and wasbroadband coated to be about 97% reflective within 900 nm-1200 nmspectral range. Considering the total distance from the chip to theoutput coupler, each of the cavities was about 23 cm long.

In the second part of the experiment, an external (outside of the lasercavity) astigmatic mode converter, or AMC, 1600 was added andjudiciously aligned with the T-cavity VECSEL. In one implementation, theAMC 1600 was based on a pair of identical cylindrical lenses 1600A,1600B separated by a distance d that was dependents on their focallengths f:

$\begin{matrix}{d = \frac{2f}{\sqrt{2}}} & (4)\end{matrix}$

In this implementation, the focal lengths of the cylindrical lenses were30 mm, causing the separation distance to be about 42 mm. To ensure thecorrect mode conversion, the incoming laser beam 1620 had to be of acertain Rayleigh range Z_(R):

$\begin{matrix}{Z_{R} = {( {1 + \frac{1}{\sqrt{2}}} )f}} & (5)\end{matrix}$

and its beam waist has to be positioned approximately in the centralpoint between those two lenses 1600A, 1600B. For this purpose, aspherical lens 1600C with focal length 150 mm was placed in front of thecylindrical lenses 1600A, 1600B (that is, between the cylindrical lensesand the output coupler 1480 of the laser cavity). The separation betweenthe elements 1480, 1600C was about 88 mm and the distance from thespherical lens 1600C to the nearer cylindrical lens 1600A was about 250mm.

Finally, the MCEs 1520, 1530 were appropriately adjusted to ensure thatthe symmetry axis of the incoming HG mode (the output 1500) be orienteddiagonally at 45° with respect to the curvature axis of the cylindricallens. The spherical lens 1600C and cylindrical lenses 1600A, 1600B wereAR coated for the spectral region of 900 nm-1200 nm to ensure that theloss introduced by the AMC 1600 be is minimal and, specifically, <0.5%in one implementation.

To confirm that the overall T-cavity 1000 is configured for simultaneousand independent generation of collinear beams at two wavelengths(forming the output 1500), the throughput 1660 through the AMC 1600 wasfiber coupled into an optical spectrum analyzer (OSA). FIG. 11 shows thelasing spectrum of the s-polarized output from the chip 1100 at about972 nm and the p-polarized output from the chip 1200 at about 1072 nm.Next, the spectral tuning range of each independent laser sub-cavity wasexaminer by rotating the BFs 1260, 1360. While each of the chips 1100,1200 was tuned individually to acquire wavelength spectra within anarrower scale on the OSA to increase the measurement resolution, thesimultaneous tuning with both BFs was also demonstrated. FIGS. 12A, 12Bprovide the plots illustrating empirically-recorded tunability for the˜970 nm and ˜1070 nm chips, respectively. In both cases the tuning rangewas ˜14 nm, while the spectral linewidth was maintained below 0.5 nm.Since the operation of the BFs 1260, 1360 does not depend on theoperation of the MCEs 1520, 1530, these tuning results remain valid forall of the HG and LG modes generated with the use of the MCEs 1520,1530. For a targeted high-order transverse mode output 1500, the powerof the output 1500 remained consistent within the spectral tuning rangeof that particular mode. As discussed in U.S. Pat. No. 9,231,373, thespectral tuning of wavelengths of light generated by the gain media1140, 1240, was optionally effectuated individually and independentlyfrom one another while allowing the corresponding wavelengths becomeequal to one another in the process of tuning.

The output powers of fundamental and higher-order transverse modesgenerated in the T-cavity VECSEL system 1000 were measured andcharacterized as shown in FIG. 13. Since the AMC setup 1600 did notintroduce any significant loss, the measured power of HG modes (presentat the output 1500 before the AMC 1600) and LG modes (present in thethroughput 1660 after the AMC 1600) were considered to be approximatelyequal. The maximum power levels of the fundamental TEM₀₀ outputs were˜3.6 W and ˜4 W (for ˜970 nm and ˜1070 nm portions of the output 1500,respectively). The HG₀₁ mode (in the output 1500) and the converted LG₀₁mode (in the throughput 1660) from chip 1100 (at 970 nm) had a maximumpower of 2.65 W. With the use of MCEs 1520, 1530, the cavity containingthe chip 1200 (operation at ˜1070 nm) generated HG₀₁ and HG₁₁ modesdelivering ˜3.4 W and ˜1.8 W of maximum power, respectively. The outputpowers of the corresponding LG₀₁ and LG₁₀ modes (measured in throughput1660) were substantially the same. The output power for the higher-ordermode output is smaller as compared to the fundamental Gaussian beamoutput due to the small amount of loss introduced by the MCEs. (Notably,the pump spot sizes remained fixed for all of the modes, thus there wasless gain provided for generation of higher-order modes, as was alreadydiscussed above). Beams carrying the HG modes (at the output 1500,without the AMC 1660 present) or LG modes (in the throughput 1600, withAMC 1660 present) generated by the chips 1100, 1200 were collinear.Thus, the total output power from the T-cavity was the sum of theindividually measured powers.

Images of various transverse modes were captured with a CCD camera (notshown in FIG. 10). To procure clear separate profiles of HG and LGmodes, first images were captured representing only one lasing chip.FIGS. 14A, 14B, 14C provide the images of such beams. The diagonal HG₀₁beams 1710, 1712 from chips 1100, 1200, respectively, were successfullyconverted to LG₀₁ modes 1714, 1716. Also, the ˜1070 nm HG₁₁ mode 1718was generated and converted to the LG₁₀ mode 1720, thereby evidencingthe generation of good-quality HG modes the proper operation of the AMC1600 to form the LG modes. To demonstrate that the laser beams from bothVECSEL samples are collinear, the images of overlapping modes wereprocured as well. FIG. 14D shows the LG₀₁ modes 1722 generated at ˜970nm and ˜1070 nm lasing simultaneously when they are collinearlypropagating, as well as in the case when they are misaligned, 1724. Theoperation of the embodiment 100 therefore demonstrated a good spatialoverlap between the outputs containing these two modes, while stillremaining the possibility to adjust their relative positions (ifrequired for certain applications). The wavelength (spectral) separationand the intensities of these two transverse modes was demonstrated to beindividually adjustable. In a related implementation, due to the VECSELopen cavity design of the setup 1000, the AMC 1600 can be incorporatedinto the laser resonator itself

Example 4

In reference to another embodiment, a laser cavity network 1800 isschematically shown in FIG. 15. The network 1800 includes two cavityaims sharing the common portion 1804. The first cavity arm additionallyincludes the free portion 1806, while the second cavity arm additionallyincludes the free portion 1808. (Thus, the laser cavity networkrepresents the T-cavity, with two cavity arms.) Portions 1806 and 1808are spatially distinct from one another, thereby providing that thefirst and second cavity arms are also spatially distinct from oneanother. Each of the first and second cavity aims includes arespectively-corresponding gain medium (VECSEL chips 1800A and 1800B, asshown), each of which, in operation, amplifies light supported by therespective cavity arm. Light portion supported and amplified by the chip1800A is referred to herein as the first light portion, while lightportion supported and amplified by the chip 1800B is referred to hereinas the second light portion. The first and second light portionspropagating intracavity within the first and second cavity arms areoverlapped (or split, depending on the direction of propagation) at thebeamsplitter BS and, when overlapped, they form a mixed beam 1809 thatcontinues its propagation (in FIG. 15—from left to right) through thecommon portion of the laser cavity network 1800 toward the opticaloutcoupler OC.

As shown, the TMCS employed by the embodiment 1800 is completelyintracavity and includes the intracavity AMC 1814 in combination withthe intracavity MCE1 (labelled as 1818A) and the intracavity MCE2(labelled as 1818B). The MCE1 is disposed in the free portion 1806 ofthe first cavity arm between the chip 1800A and the beamsplitter BS,while the MCE2 is disposed in the free portion 1808 of the second cavityarm between the chip 1800B and the beamsplitter BS. The MCE1 and MC2 areappropriately structured (as discussed below) to ensure that lightdistributions amplified by chips 1800A, 1800B are represented by atleast one of the lowest order Gaussian mode (labelled G1, G2) and/or atleast one of the Hermit-Gaussian modes (labelled HG1, HG2). Accordingly,the mixed beam 1809 has a transverse distribution of light characterizedby combination of at least some of the Gaussian and Hermite-Gaussianmodes defined within the first and second free portions 1806, 1808 ofthe individual first and second cavity arms.

The AMC portion 1814 of the TMCS is shown positioned across the opticalaxis intracavity in the common portion 1804 of the laser cavity network1800. Configured with the use of refractive optical elements configuredto change angle(s) of spatial divergence of the beams 1809, 1809′, theAMC 1814 operates on the mixed beam 1809 such that, upon traversing theAMC 1814 (as shown—from left to right) the mixed beam 1809 istransformed to a mixed beam 1809′ the wavefront of which is representedby Laguerre-Gaussian modes (LG1 that correspond to the HG 1 modes of thefree portion 1806 of the first cavity arm, and/or LG2 that correspond tothe HG2 modes of the free portion 1808 of the second cavity arm).

The outcoupler OC is disposed to form an optical fold 1810 of and withinthe common portion 1804 of the laser cavity network 1800, and in aspecific case the OC can be structured as a curved optical reflector. Atleast a part of light energy contained in the mixed beam 1809′ isreflected by the OC into the folded part 1810 of the common portion 1804towards the end reflector ER. The end reflector ER is generallypositioned at or in the vicinity of location of the waist of theRayleigh region defined by the transverse distribution of light directedtowards it intracavity by the outcoupler OC. The non-linear optical(NLO) element such as a non-linear crystal is disposed between the ERand the OC to ensure that at least some light energy contained in lightdistribution of the beam 1809′ that arrived at the OC from the BS andthat has been reflected by the OS towards the ER passes through theNLO-element and is non-linearly converted to light at a differentoptical frequency (as governed by specific circumstances describing theparticular implementation of the embodiment 1800). The spectraldistribution of reflectance characteristics of the OC is judiciouslydefined to provide for at least a partial transmission of light atwavelenghths (frequencies) resulting from the NL-conversion of lightintracavity at the NLO-element, such that at least a portion of light Loutcoupled from the cavity network 1800 through the OC (and forming thesought-after laser output) has an optical spectrum including thewavelengths generated as a result of interaction between the NLO-elementand light from the beam 1809′. At the same time, as is readilyunderstood, the transverse distribution of the light output L includesLG modes present in the mixed beam 1809′. Light output L includes LGmodes of the nonlinearly mixed beams generated from the beams 1809. (TheSFG or DFG of different LG modes generates a new LG mode at a newfrequency).

In a specific implementation, an auxiliary light output L′ from thelaser cavity network 1800 can be formed through the ER by making itpartially transparent to light at wavelengths present at point P betweenthe NLO element and the ER. For example, the optical properties of thereflectors OC and ER can be defined to outcouple the non-linearlyconverted light at SFG/DFG frequenc(ies) through the OC whileoutcoupling at least a portion of light at least one of the wavelengthsλ₁, λ₂ through the ER.

Example 5

In a related implementation 1900, shown schematically in FIG. 16, onecavity arm of the 2-arm T-cavity-based laser-cavity network can beconfigured to be passive in that it is structured to not contain anygain medium (see arm portion 1910, defined between the cavity reflector1916 and the PBS 1934) and to deliver external pump energy into thelaser cavity via a low-power beam 1912, generated by the pump source1914 and transmitted through the reflector 1916. The free-portion of thecavity arm 1920 containing the gain medium of chip 1900A also includesan MCE 1924, while the AMC 1928 is disposed across the optical axis 1930between the PBS 1934 and the curved reflector 1938 (light outcoupler).The common collinear cavity portion of the laser network 1900 (definedbetween the PBS 1934 and the planar reflector 1939) is denoted as 1940.It includes a “fold” 1944 of the cavity, containing the nonlinearfrequency conversion element 1946. This configuration offers substantialfield enhancement and control of the transverse mode distribution (Theexternal low power will have the desired mode for the mode control. Ahigh-reflective mirror place after the external source and the flatmirror next to the nonlinear crystal creates a high-Q Fabry-Perot cavityresulting in significant field enhancement of the external source. As aresult the configuration will offer field enhancement and mode control.)In this case, the generated laser output 1950 represents a high-powerversion of the incident laser beam 1912 at the desired frequency ofchoice. The transverse properties of light 1940 that has been outcoupledfrom the laser-cavity network through the light outcoupler 1938 aredefined by the operation of the AMC 1928 on transverse modes (defined bythe MCE 1924 in the cavity arm 1920), by analogy with the operationdescribed above in reference to embodiment 1800 of Example 4.

Example 6

FIG. 17 shows a schematic 2000 of a two-arm T-cavity laser cavitynetwork, in which one arm 2010 (containing the gain medium, VECSEL chip2000A) is configured to independently lase on a fundamental Gaussianmode, while another arm 2020 (containing the gain medium, VECSEL chip2000B) is configured to independently lase on the HG₀₁ mode (in favor ofwhich the transverse distribution of light is discriminated by an MCE2024, configured as a wire unit and disposed intracavity across the beam2026 and the optical axis of the arm 2020). None of the components ofthe intracavity TMCS is shown for simplicity of illustration. Thecombination of these two modes, that are simultaneously present in amixed beam 2030 propagating through the common collinear portion of thecavity network and traversing the nonlinear crystal 2034 (located in thefold 2036) upon the reflection off the curved mirror 2040, causes thegeneration of an HG₀₁ mode beam (for example, at a second harmonicfrequency as a result of non-linear frequency conversion). This SH lightis transmitted as the output 2050 through the outcoupler 1938 (at leastsemitransparent at the frequency of the output 2050). The external modeconverter 2054 (composed, for example, of a pair of cylindrical lensesand shown for simplicity of illustration as a single optical element)turns the HG₀₁ mode into an LG₀₁ mode upon the traversal of the output2050 through the converter, thereby forming a throughput 2060.Higher-order modes can be generated in a similar manner. It would bereadily appreciated by a skilled artisan that when, by analogy with theembodiment of Example 4, MCE1 and MCE2 are configured as wiresorthogonal to one another are placed in each of the cavity arms 2010,2020, one of the gain-medium chips 2000A, 2000B is enabled to operate inan HG₀₁ mode, while the other of these gain-medium chips can operate ina HG₁₀ mode.

Generally, an MCE employed in any embodiment of the invention isconfigured to contain at least one MCE element. The MCE element, in itssimplest form, may be represented by a wire or a trace of a lossymaterial (formed, for example, lithographically) on a substantiallylossless substrate that is disposed across the optical axis intracavityto provide for different level of optical losses for light at differenttransverse modes (i.e., providing for non-uniform distribution ofoptical losses in a plane across the axis of the laser cavity) and,therefore, preferentially create conditions for amplification of thosetransverse modes that experience lower losses intracavity. In a relatedembodiment, an MCE may include an element formed by at least two“wires”, which are disposed parallel to one another or that cross oneanother. In yet another related embodiment, an MCE includes asuperposition of wires forming a wire-mesh, in which multiple wires areextending, in a given plane, in one direction substantially parallel toone another and another set of wires extends in a transverse direction(in the same given plane or in a plane parallel to the given plane). Inyet another embodiment, an MCE unit is used that is structured as anarray (for example, a 1D-array or a 2D-array containing any feasiblecombination of the above-mentioned MCE elements.

In light of the discussion presented above, a person of ordinary skillin the art will readily appreciate that an intracavity beam-mixingprocess, resulting in the formation of a desired transverse-modeprofile, is within the scope of the invention. Such intracavity beammixing (and corresponding hardware set-up) includes:

A) Generating light with a wavefront having a spatial distribution thatincludes at least one desired HG mode in a cavity arm containing again-medium with the use of either (i) a corresponding MCE (a portion ofthe TMCS) disposed in such cavity arm or (ii) an off-axis pumping of thegain medium. When multiple cavity arms containrespectively-corresponding gain media (VECSEL chips), multiple HG modesare being formed in the laser cavity network, each at a wavelength oflight amplified by the corresponding gain medium in a given cavity arm.In a specific case a simple Gaussian mode can be generated, either aloneor in combination with another HG mode.

B) Passing the so-generated light intracavity through an AMC (anotherportion of the TMCS) to transform the generated HG mode(s) to the LGmode(s). The traversal of the AMC leaves the simple Gaussian modeunchanged, intact.

C) Passing the light contained in LG mode(s) intracavity through a NLOelement to generate, intracavity, at least one of SHG-light, SFG-light,and DFG-light in LG transverse mode(s).

The choice of HG modes and associated LG modes and their combination(s)provides a wide range of LG modes in the laser output extracted from thecavity network through the output coupler at the targeted wavelength(s).In an embodiment configured to provide a spectrally-tunable laser outputcontaining at least two different wavelengths of light (such asembodiment 1800 of FIG. 15, for example), the generation ofsubstantially any targeted wavelength may be implemented through thenon-linear conversion of optical frequencies, thereby facilitating thegeneration of a spectrally-tunable LG-mode based laser output.

The spectral tuning of operation of any of the cavity arms of anembodiment of the invention is carried out with the use of a processdiscussed in U.S. Pat. No. 9,231,373, which (in a specificimplementation) causes the tuning of light at first and secondwavelengths (respectively generated by and supported by different cavityarms) to be independent from one another while allowing these twowavelengths to become equal to one another during the tuning process.(Such process, advantageously, has little or no effect on the operationof the components of the TMCS of the device of the invention, and forthat reason is intended to be freely implemented.) The AMC portion ofthe TMCS may include a system of lenses and is substantially lossless atwavelengths of resonance of the cavity network. The effect of the MCE onlight traversing it has also minimal effect on the cavity loss. As aresult, the laser output in the form of spectrally—and/or spatiallytunable/variable HG and/or LG modes is formed with inherently highpower, comparable to that characterizing performance of a conventionalT-cavity VECSEL-based laser system.

It is understood therefore that, generally, the partially co-linear opencavity network of the proposed device for multiple wavelength/multipletransverse mode generation is built around a cavity design that hasmultiple spatial cavity sleeves or branches or arms extendedtransversely with respect to one another and sharing a co-linear cavityportion, as discussed, for example, in the co-assigned U.S. Pat. No.9,231,373, the disclosure of which is incorporated herein by referencefor all purposes. Each of the branches includes a corresponding gainelement (optionally-VECSEL chip) within the cavity. In one embodiment, aVECSEL chip may contain semiconductor quantum wells judiciouslyengineered to support light generation within large wavelength range(670nm-2.4 μm has been empirically demonstrated).

Example 7

Here, the components of the TMCS (that is, MCE(s) and AMC(s)) are usedto complement the device structure described in reference to FIG. 5 ofU.S. Pat. No. 9,231,373, reproduced herein as FIG. 18. FIG. 18 is adiagram schematically illustrating an embodiment of the co-linearT-cavity VECSEL device of the invention structured for type-II sumfrequency generation or difference frequency generation. A setup 2100 isschematically shown to include a two-chip co-linear T-cavity VECSELdevice that folds a portion of intracavity light 2138 (s-polarized)about a polarizing beamsplitter 2144 while allowing the second portionof light 2128 (p-polarized) to pass through the splitter 2144 unabated.This implementation lends itself to implementing a tunable intracavitytype II sum frequency generation (SFG) to achieve high-power, highstability light output 2150 in the blue-green spectral bands. TheT-cavity includes three spatially distinct cavity regions or portions orsections 2110, 2120, 2510 each of which supports VECSEL's lasing in adistinct fashion As was discussed in reference to FIG. 15, the spatiallydistinct cavity portions 2110, 2120 are structured to supportindependent lasing at wavelengths λ₁, λ₂ and at orthogonalpolarizations, while the collinear cavity section 2510 is shared by (iscommon to) intracavity light portions 2128, 2138. As a result,independent generation and tuning of each lasing wavelength λ₁, λ₂ isenabled over a wide range defined by different semiconductor gain bandscorresponding to different gain media (VECSEL chips) 2114, 2124.

The collinear cavity portion 2510 (defined by the intracavity opticalpath from the PBS 2144 to the output coupler 2512 to thehigh-reflectance optionally flat mirror 2514) includes a cavity fold2540. The output coupler 2512 is appropriately shaped and positionedsuch as to form a Rayleigh region of an intracavity light beampropagating between the output couple 2512 and the mirror 2514substantially in the region of the fold 2540 itself, where thenon-linear optical medium 2526 can be placed (in which case the cavitymodes lasing at λ₁, λ₂ spatially overlap at the medium 2526). Theformation of the cavity fold 2540 allows the light at the fundamentalwavelengths λ₁, λ₂ to be substantially overlapping and co-propagatingalong the corresponding Rayleigh ranges to maximize the efficiency ofthe non-linear frequency conversion. This flexibility in beam sizecontrol facilitates efficient sum or difference frequency generation inthe embodiment of the device. (Similar folding of a collinear cavity canbe employed with any of other embodiments of the invention.)

While not shown in FIG. 18 for simplicity of illustration, at least oneMCE were disposed in individual (not shared) portions of the aims of thecavity network (for example, between the elements 2124 and 2144, and/orbetween the elements 2114 and 2144, while the AMC was disposed withinthe common, shared, collinear portion 2510, to carry out thetransformation of transverse modes within the cavity network accordingto the idea of the invention.

In one implementation ofthe embodiment 2100, strain compensatedInGaAs/GaAs/GaAsP multi-quantum well (MQW) structures capable ofemitting light in the proximity of 980 nm were used. The “bottomemitting” VECSEL structure included an active region containing 14 QWs(each of about 8 nm thick), surrounded by GaAsP strain compensationlayers and GaAs pump absorbing barriers. A high reflectivity (R˜99.9%)DBR mirror made of alternating Al_(0.2)Ga_(0.8)As/AlAs was grown on topof such MQW structure. The thickness and composition of the layers weredesigned such that each QW be positioned at an antinode of the cavitystanding wave to provide resonant periodic gain (RPG). In order tofacilitate selective substrate etching process, a thin high aluminumconcentration AlGaAs etch-stop layer is initially grown on the GaAssubstrate prior to the active layers growth. The VECSEL structurefabrication process included solder-bonding the epitaxial side of thewafer on a high thermal conductivity chemical vapor deposition (CVD)diamond followed by substrate removal through a selective wet etchingprocess. (See, for example, C. Hessenius et al., in Proc. Of SPIE, v.8242, 82420E, 2012, incorporated herein by reference). The processeddevices were mechanically mounted on a water-cooled copper heat sink fortemperature control.

In practice, a pump spot diameter of ˜500 microns was used on each ofthe chips 2114, 2124. (It is understood, however, that the same ordifferent appropriately chosen pump-spot-diameter pump beams can beemployed, for example as large as 1 to 2 mm, in some cases). Thedistance from the surface of each of the chips 2114, 2124 to the curvedOC 2512 was about 19 cm and the distance from the curved OC 2512 to theHR flat mirror 2514 was about 6 cm. The temperature of the gain media2114, 2124 was maintained at about 15 C with the use of heatsinks 122.The flat end mirror 2514 incorporated a broadband HR coating, while thecoating at the surface of the curved OC 2512 exhibited high reflectance(of about 99.9%) at a fundamental lasing wavelength and low reflectance(for example, less than 0.25%) for the blue-green portion of the lightoutput 2550. It is understood that when the two VECSEL chips 2114, 2124are chosen from the same wafer growth, very spectrally close or nearlyidentical gain curves and, therefore, gain center wavelengths can beprovided to support the intracavity optical fields 2128, 2138. Thespectral separation between the wavelengths λ₁, λ₂ is adjusted bycontrolling the angular orientation of the BF(s) 2126, 2136 rotation andindividual wavelength tuning of each polarization.

Example 8

In this case, the idea of the invention is implemented by supplementingthe device structure described in reference to FIG. 10 of U.S. Pat. No.9,231,373 (reproduced herein as FIG. 19) with the TMCS according to theidea of the invention. FIG. 19 is a diagram illustrating a T-cavityVECSEL structured to generate the UV light (optionally, in addition tothe generation of the IR and/or visible light). In reference to FIG. 19,an embodiment 2600 structured based on the modification of theembodiment 2100 of FIG. 18, for example, employs the gain medium 2600Aand two non-linear media elements 2628 (in the fold 2622 of theT-cavity, terminated with the reflector 2629) and 2630 in the cavity arm2634. The element 2628 is chosen for non-linear conversion of light atIR wavelength λ₁ to visible light (at least one of which can beoptionally extracted from the T-cavity through the appropriately coatedoutput coupler 2650). For example, when 980 nm, the type I SHG in theelement 2628 can be realized to generate SH light at about 490 nm andpolarization that is orthogonal to that of the light at 4 Theintracavity circulation of the SH optical field is further supported bya portion of the T-cavity including the arm 2634 that is terminated witha mirror 2640 in order to generate, with the use of the non-linearelement 2630, the UV output 2654 (for example, at about 245 nm)extracted through the output coupler 2660. (It is understood that, whilethe PBS 2662 is shown, the non-polarizing semitransparent mirror can beused instead depending on whether it is desired to have differentintracavity optical fields be similarly or orthogonally polarized.Spectral tenability of the system can be realized, for example, with theuse of an angularly re-adjustable BF, not shown for simplicity ofillustration. While not shown in FIG. 19 explicitly, an MCE was disposedin the individual (not shared) portion of the active arm of the cavitynetwork (for example, between the elements 2660A and 2662), whilepositions for AMC included the cavity extents between the elements 2662and 2644; and/or 2630 and 2662 to effectuate the transformation oftransverse modes within the cavity network according to the idea of theinvention.

While the implementations of the embodiments of the invention weredescribed in reference to specific diagrams and laser cavity networks,it is understood that any cavity configuration described in U.S. Pat.No. 9,231,373 can be used in implementation of the idea of the presetinvention as long as at least one of the individual cavity arms of theoverall cavity network contains a TMCS. According to the idea of theinvention, an MCE component of the TMCS is present in at least onecavity arm while an AMC component of the TMCS is also present in atleast one cavity arm, intracavity. In so structuring an embodiment ofthe invention, the embodiment may be further complemented with at leastone wavelength tuning mechanism (such as a rotatable BF) juxtaposed withthe cavity network and configured to tune a first wavelength of multiplewavelengths generated by the embodiment independently with respect to asecond wavelength of such multiple wavelengths while allowing the firstand second wavelengths to become equal as a result of tuning.

Finally, as discussed in U.S. Pat. No. 9,231,373, in each of variousembodiments, pulsed operation of the beams at fundamental frequencies(as opposed to those at non-linearly converted frequencies) can beincorporated into the setup. By the means of passive mode-locking (e.g.SESAM) or q-switching (e.g. chopper) systems juxtaposed with theembodiment of the laser source, one or both of the input waves can bepulsed, leading to space-time novel laser fields. For example,Airy-Bessel beams, that have an Airy temporal profile and a Besselspatial profile, have been proposed as versatile linear light bullets.Accordingly, embodiments can be used to generate the full variety ofsuch space-time novel laser pulses.

Overall, a skilled artisan will readily appreciate that a laser source,configured according to the idea of the invention, comprises a lasercavity network including first and second spatially-distinct cavity armsand a collinear portion, wherein the first and second spatially-distinctcavity arms share the collinear portion. (At least one of the first andsecond cavity arms contain, intracavity, a corresponding gain mediumincluding one of a VECSEL-based laser gain medium, a solid-state gainmedium, and a fiber amplifier and configured to provide amplification oflight at a corresponding wavelength). The laser source also comprises afirst optical system (disposed across an axis of the at least one of thefirst and second cavity arms, to either refract or reflect lightincident thereon while transforming a transverse distribution of saidlight) a second optical system (disposed across said axis andcharacterized by optical losses, at the corresponding wavelength, thatare non-uniformly distributed across the second optical system). Thesecond optical system disposed between the corresponding gain medium andthe first optical system. Optionally, both the first and the secondoptical systems may be located in the collinear portion of the lasercavity network.

The so-configured laser source in operation, maintains intracavitygeneration of light at the corresponding wavelength, said light having,intracavity, (i) a first transverse mode distribution in a first portionof the laser cavity network between the gain medium and the secondoptical system, (ii) a second transverse mode distribution in a secondportion of the laser cavity network between the second and first opticalsystems, and (iii) a third transverse mode distribution in a thirdportion of the laser cavity network, the third portion being a remainingportion of the laser cavity network, the first, second, and thirdtransverse mode distributions being different from one another.

A laser source further includes an optical element defining a spatialfold of a path of light propagating intracavity and forming,intracavity, first and second Rayleigh regions of first and second lightportions of light, where the first and second light portionsrespectively contain the first and second wavelengths, and where thefirst and second Rayleigh regions overlap with one another in thespatial fold. Here, at least one of the following conditions issatisfied: a) such optical element is not a reflector that terminatesthe cavity network while such optical element is an output couplerconfigured to transmit a portion of light, generated intracavity, toform a light output from the laser source, and b) at least one of thefirst and second cavity arms contains an intracavity non-linearfrequency conversion element. In a specific implementation, at least oneof the following conditions may be satisfied: (i) at least one of thefirst and second cavity arms is devoid of a gain medium, and (ii) thefirst transverse mode distribution is a Gaussian distribution; thesecond transverse mode distribution is a Hermite-Gaussian distribution,and the third transverse mode distribution is a Laguerre-Gaussiandistribution. Alternatively, each of the first and second cavity armsmay contain a respectively corresponding first and second VECSEL-basedlaser medium to cause independent lasing at respectively-correspondingfirst and second wavelengths, while the laser source further comprisesat least one wavelength tuning mechanism juxtaposed with the cavitynetwork and configured to tune at least one of the first and secondwavelengths independently from another of the first and secondwavelengths while allowing the first and second wavelengths to becomeequal as a result of such tuning.

Depending on the embodiment, the first cavity arm may contain acorresponding gain medium element while the second cavity arm may bedevoid of a corresponding gain medium element. In this case, anend-reflector of the second cavity arm is made partially transparent tolight at a pump wavelength, the laser source is further equipped with apump laser source that is configured to generate pump light at said pumpwavelength and is positioned to deliver said pump light through said endreflector along an optical axis of the second cavity arm. Depending onthe embodiment, The AMC may be disposed inside or outside the multi-armcavity network. Alternatively or in addition (and while not necessarilyshown in the drawings, for simplicity of illustrations), more than oneAMC may be disposed inside at least one cavity arm of the multi-armcavity network of the system of the invention.

For the purposes of this disclosure and the appended claims, the use ofthe terms “substantially”, “approximately”, “about” and similar terms inreference to a descriptor of a value, element, property orcharacteristic at hand is intended to emphasize that the value, element,property, or characteristic referred to, while not necessarily beingexactly as stated, would nevertheless be considered, for practicalpurposes, as stated by a person of skill in the art. These terms, asapplied to a specified characteristic or quality descriptor means“mostly”, “mainly”, “considerably”, “by and large”, “essentially”, “togreat or significant extent”, “largely but not necessarily wholly thesame” such as to reasonably denote language of approximation anddescribe the specified characteristic or descriptor so that its scopewould be understood by a person of ordinary skill in the art. The use ofthese terms in describing a chosen characteristic or concept neitherimplies nor provides any basis for indefiniteness and for adding anumerical limitation to the specified characteristic or descriptor. Asunderstood by a skilled artisan, the practical deviation of the exactvalue or characteristic of such value, element, or property from thatstated falls and may vary within a numerical range defined by anexperimental measurement error that is typical when using a measurementmethod accepted in the art for such purposes.

An embodiment of the system of the invention includes electroniccircuitry (for example, a computer processor) controlled by instructionsstored in a memory, to perform governing of the described laser sourceand/or specific data collection/processing steps. The memory may berandom access memory (RAM), read-only memory (ROM), flash memory or anyother memory, or combination thereof, suitable for storing controlsoftware or other instructions and data. Those skilled in the art wouldreadily appreciate that instructions or programs defining the operationof the present invention may be delivered to a processor in many forms,including, but not limited to, information permanently stored onnon-writable storage media (e.g. read-only memory devices within acomputer, such as ROM, or devices readable by a computer I/O attachment,such as CD-ROM or DVD disks), information alterably stored on writablestorage media (e.g. floppy disks, removable flash memory and harddrives) or information conveyed to a computer through communicationmedia, including wired or wireless computer networks. In addition, whilethe invention may be embodied in software, the functions necessary toimplement a method of the invention may optionally or alternatively beembodied in part or in whole using firmware and/or hardware components,such as combinatorial logic, Application Specific Integrated Circuits(ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware orsome combination of hardware, software and/or firmware components.

The idea and implementation of the idea of the present invention opensthe door to obtaining high-power circulating within the laser cavitynetwork LG modes, which opens the path to producing circularly symmetrictransverse modes at multiple wavelengths by the means of nonlinearfrequency conversion, as discussed in Examples above. The currentlylimited availability of higher order transverse modes in spectralregions such as UV, visible or mid-IR is, therefore, greatly expanded.Thus, the laser systems configured according to the idea of theinvention, thanks to their flexibility and opportunities forcustomization, can be implemented when beams with particular orbitalangular momentum are required for atoms or particles manipulation.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole. Variouschanges in the details, steps and components that have been describedmay be made by those skilled in the art within the principles and scopeof the invention.

1. A laser source comprising: a laser cavity network including first andsecond spatially-distinct cavity arms and a collinear portion, whereinthe first and second spatially-distinct cavity arms share the collinearportion, at least one of the first and second cavity arms containing acorresponding gain medium that includes one of (i) a VECSEL-based lasergain medium, (ii) a solid-state gain medium, and (iii) a fiberamplifier, and that is configured to provide amplification of light at acorresponding wavelength; and at least one of a first transverse lightmode converter optical system, disposed across an axis of the at leastone of the first and second cavity arms to either refract or reflectlight incident thereon, and a second transverse light mode converteroptical system disposed across said axis between the corresponding gainmedium and a terminus of the collinear portion; wherein said lasersource is configured to maintain intracavity generation of light at thecorresponding wavelength such that a first transverse light modedistribution is formed between the second transverse light modeconverter and the collinear portion and a second transverse light modedistribution is formed in a remaining portion of the laser cavitynetwork, and wherein the first transverse light mode distribution andthe second transverse light distribution differ from one another.
 2. Alaser source according to claim 1, comprising a non-linear opticalconversion element at least in the collinear portion of the laser cavitynetwork.
 3. A laser source according to claim 1, further comprising anoptical element defining a spatial fold in a path of light propagatingintracavity and forming, intracavity, first and second Rayleigh regions,respectively, of a first light portion of said light and a second lightportion of said light, wherein the first light portion and the secondlight portion respectively contain a first wavelength and a secondwavelength, wherein the first light portion is defined and amplified bythe corresponding gain medium, and wherein the second light portion isdefined by interaction of the first light portion with a non-linearoptical conversion element of the laser cavity network.
 4. A lasersource according to claim 3, wherein at least one of the followingconditions is satisfied: a) said optical element is not a reflector thatterminates the cavity network while said optical element is a firstoutput coupler configured to transmit a portion of light, generatedintracavity, to form a first light output from the laser source, thefirst light output containing light from the second light portion; andb) at least one of said first and second cavity arms contains anauxiliary intracavity non-linear frequency conversion element.
 5. Alaser source according to claim 3, wherein at least one of the followingconditions is satisfied: a) only one of the first cavity arm and thesecond cavity arm contains the corresponding gain medium; b) only one ofthe first cavity arm and the second cavity arms contains the secondtransverse light mode converter optical system; and c) the firsttransverse light mode converter optical system is disposed outside ofthe laser cavity network to receive light from the second light portiontransmitted through said optical element.
 6. A laser source according toclaim 1, wherein the at least one of the first and second cavity armsincludes a mirror terminating the at least one of the first and secondarms, wherein the at least one of the first and second cavity arms isdevoid of the corresponding gain medium, and wherein the at least one ofthe first and second cavity arms includes an additional output couplerthat is disposed between the mirror and the collinear portion to form asecond light output therethrough, the second light output having a thirdwavelength.
 7. A laser source according to claim 1, wherein the firstcavity arm contains a gain medium element while the second cavity arm isdevoid of any gain medium element, wherein an end-reflector of the firstcavity arm is partially transparent to light at a pump wavelength, andfurther comprising a pump laser source that is configured to generatepump light at said pump wavelength and is positioned to deliver saidpump light through said end-reflector along an optical axis of the firstcavity arm.
 8. A laser source according to claim 1, wherein said secondtransverse mode converter optical system is characterized by opticallosses that are non-uniformly distributed across the second transversemode converter optical system.
 9. A laser source according to claim 1,wherein first and second portions of the laser cavity network, in whichthe respective first and second transverse mode distributions areformed, are separated from one another by a refractive optical elementdisposed across an axis of the laser cavity network.
 10. A laser sourceaccording to claim 1, wherein at least one of the following conditionsis satisfied: a) at least one of the first and second cavity arms isdevoid of any gain medium; and b) wherein the first transverse modedistribution is a Gaussian distribution; the second transverse modedistribution includes at least one of a Hermite-Gaussian distributionand a Laguerre-Gaussian distribution.
 11. A laser source according toclaim 1, wherein at least one of the following conditions is satisfied:a) the second transverse mode converter optical system is characterizedby optical losses that are non-uniformly distributed across the secondtransverse mode converter optical system; and b) the laser sourcefurther comprises a non-linear optical conversion element in thecollinear portion of the laser cavity network.
 12. A method comprising:forming a first intracavity standing optical wave, at a firstwavelength, between and terminated by a first reflector and a secondreflector of a laser cavity network, wherein said laser cavity networkincludes multiple spatially-distinct cavity arms that share a collinearportion of the laser cavity network, wherein said first intracavitystanding optical wave has a first transverse mode distribution in thecollinear portion of the laser cavity network and a second transversemode distribution in a remaining portion of the laser cavity network;wherein the first and second transverse mode distributions are differentfrom one another; and outcoupling a pre-determined portion of lightenergy, provided by said first intracavity standing optical wave,through an outcoupler of the laser cavity network, wherein saidpre-determined portion of light energy has at least one of the firstwavelength and a second wavelength, the first wavelength and the secondwavelength being different from one another.
 13. A method according toclaim 12, wherein one of the following conditions is satisfied: (a) saidoutcoupler is a reflector separated from the collinear portion by anoptical component of the laser cavity network and said pre-determinedportion of light energy has the second transverse mode distribution; and(b) said outcoupler is a reflector located in the collinear portion andsaid pre-determined portion of light energy has the first transversemode distribution.
 14. A method according to claim 12, wherein themethod further comprises: forming intracavity light at the secondwavelength, wherein the second wavelength is defined as a result of anon-linear frequency conversion of said predetermined portion of lightenergy inside the laser cavity network, wherein the outcoupling includesoutcoupling of light at the second wavelength; and wherein theoutcoupler is a reflector located in the collinear portion.
 15. A methodaccording to claim 14, wherein first and second portions of the lasercavity network, in which the first intracavity standing optical wave hasrespective first and second transverse mode distributions, are separatedfrom one another by a refractive optical element disposed across thefirst intracavity standing optical wave.
 16. A method according to claim14, wherein at least a part of the second portion of the laser cavitynetwork belongs to only one of said multiple spatially-distinct cavityarms.
 17. A method according to claim 12, further comprising at leastone of (a) amplifying light at said first wavelength contained in thefirst intracavity standing wave with a first gain medium contained inthe first cavity arm of the laser cavity network; and (b) forming asecond intracavity standing optical wave, at a second wavelength,between and terminated by a third reflector and the second reflector ofthe laser cavity network, wherein said second intracavity standingoptical wave has a fourth transverse mode distribution in the collinearportion of the laser cavity network and a fifth transverse modedistribution outside of the collinear portion of the laser cavitynetwork; wherein the fourth and fifth transverse mode distributions aredifferent from one another.
 18. A method according to claim 17, furthercomprising transmitting the second intracavity standing optical wavethrough a non-linear optical element disposed between the second andthird reflectors to generate laser energy at a third wavelength that isdifferent from the first wavelength and the second wavelength; andoutcoupling said laser energy at the third wavelength through a partialreflector that has a curved reflecting surface and that is separatedfrom the third reflector by said non-linear optical element.
 19. Amethod according to claim 12, wherein the forming includes forming saidfirst intracavity standing optical wave that has a sixth transverse modedistribution in a second part of the first arm, said second part of thefirst arm located outside of the collinear portion of the laser cavitynetwork, said second part of the first arm being different from thefirst part of the first arm; wherein the sixth and second transversemode distributions are different from one another.
 20. A methodaccording to claim 12, wherein at least one of the following conditionsis satisfied: a) the method comprises changing at least one of the firstand second transverse mode distributions by respective intracavityrepositioning of at least one of first and second optical elementsdisposed between the first and second reflectors; and b) wherein saidforming occurs simultaneously with said changing and said outcoupling.